The present invention relates to a video tape recorder, and more particularly to the recording and reproduction of display data in digital form on a helically scanned magnetic tape.
Analog-to-digital conversion techniques are currently employed for high quality sound recording. In the standard 8-mm video tape recorder, tape is wound on a rotary drum over 180 degrees plus additional 36 degrees for recording digitized audio signals in the 36-degree additional sector of the track and recording analog video signals on the 180-degree sector, as illustrated in FIG. 1. Also known in the art is a VTR shown in FIG. 2 in which each of the skewed tracks is divided into six sectors and digitized multi-channel signals are recorded on respective sectors. Specifically, audio signal from a first sound channel is digitized into digital samples and sequentially recorded on sectors BL.sub.1 -A and BL.sub.1 -B and the digital samples of a second channel are sequentially recorded on sectors BL.sub.2 -A and BL.sub.2 -B, and so on, with the sectors BL.sub.1 being recorded by transporting the tape in a forward direction and the sectors BL.sub.2 being recorded by reversing the direction of tape motion at the tape end. Long-play mode is possible by transporting the tape at a reduced speed.
FIG. 3 is an illustration of a recording format for recording digitized multi-channel signals by transporting the tape in opposite directions at a low speed for "standard play" and at a high speed for "long play". With the video tape recorders of this type which permits recording of multiple source data, the user is allowed to select one or more desired operating modes and record identification words in specified areas of the tape to be detected automatically on playback in order to determine the tape speed according to the detected identification word, so that the tape speed can be automatically adjusted to the specified speed. It is desired to record visual information such as program numbers and time data at intervals. However, it is impossible to record all the visual information in the space currently available.
One approach is to divide such data into an M-bit header, or discrimination word and an (N-M)-bit control word as shown in FIG. 4 or organize M blocks of N bits each, each block forming an identification word as shown in FIG. 5. In the standard 8-mm video tape recorder 6 bytes are assigned to identification words designated ID.sub.O through ID.sub.5, respectively, with the word ID.sub.5 being used as a control signal having a format as shown in FIG. 6. Word ID.sub.0 is a header, or discrimination word designated "mode number" which identifies a particular combination of the other identification words ID.sub.1 through ID.sub.5. Currently, six different discrimination words are available, two examples of which are shown in FIGS. 7 and 8.
FIG. 9 shows a standard format for 8-mm video tape recorders. In the NTSC system, the data to be recorded on each track comprises 132 blocks of sync, address, P and Q parities, 8-word data signals and a 16-bit CRC (cyclic redundant check) word. The identification words ID.sub.O through ID.sub.5 are recorded in different parts of the track. Specifically, ID.sub.0 to ID.sub.5 are recorded on the 1st, 2nd, 45th, 46th, 89th and 90th blocks, respectively (shown at a, b, c, d, e and f). However, during search modes the tape is transported at higher speed than normal and as a result the video head crosses several tracks at different locations of each track. Since the identification words are recorded at spaced intervals, the discrimination word ID.sub.0 cannot be detected with other identification words in a single high-speed scan. The loss of the discrimination word results in a failure to discriminate other identification words which may be detected from different tracks.